The present invention relates to a wire electric-discharge machining apparatus.
FIG. 8 is a block diagram that shows a wire electric-discharge machining apparatus based on a conventional art of this type. In FIG. 8, reference numeral 1 denotes a wire electrode, 2 a workpiece, 3 a wire supplying unit, 4 a dielectric fluid supplying unit, 5 a machining power supply, 6 an average voltage measurement unit, 7 a control parameter setting unit, 8 a relative feed rate determining unit, 9 a control unit, and 10 a driving unit.
The wire supplying unit 3 feeds out the wire electrode 1 at an appropriate speed, and allows this to travel while giving an appropriate tension to the wire electrode 1. The dielectric fluid supplying unit 4 supplies a dielectric fluid in a minute gap between the wire electrode land the workpiece 2. The machining power supply 5 applies a pulse-like voltage between the wire electrode 1 and the workpiece 2 so that a discharge is generated between the wire electrode 1 and the workpiece 2. The average voltage measurement unit 6 measures an average voltage between the wire electrode 1 and the workpiece 2. The control parameter setting unit 7 sets a reference voltage and a target feed rate based on machining conditions set by the user. The relative feed rate determining unit 8 calculates a relative feed rate between the wire electrode 1 and the workpiece 2 by using the average voltage measured by the average voltage measurement unit 6 and the reference voltage and the target feed rate set by the control parameter setting unit 7 to supply the relative feed rate to the control unit 9. The control unit 9 relatively moves the wire electrode 1 and the workpiece 2 at the relative feed rate thus calculated through the driving unit 10.
The sequence of calculating the relative feed rate between the wire electrode 1 and the workpiece 2 in the relative feed rate determining unit 8 will be explained below.
First, the relative feed rate determining unit 8 compares the measured average voltage obtained by the average voltage measurement unit 6 with a preset short circuit reference voltage. The short circuit reference voltage is a voltage used as a reference when a case in which the measured average voltage is below this voltage is determined that the generation of a discharge is impossible because the wire electrode 1 and the workpiece 2 are in contact with each other. For example, when the wire electrode 1 is made of brass with the workpiece 2 being made of steel, the above-mentioned relative feed rate determining unit 8 generally sets the short circuit reference voltage to a value in the range of 10 to 15 V.
As a result of comparison between the measured average voltage and the short circuit reference voltage, when the measured average voltage goes below the short circuit reference voltage, the relative feed rate determining unit 8 sets the relative feed rate to a comparatively great negative value. Consequently, the wire electrode 1 and the workpiece 2 are separated from each other at a high speed, thereby making it possible to eliminate a short circuit state between them.
On the other hand, when the measured average voltage is not less than the short circuit reference voltage, the relative feed rate determining unit 8 sets the relative feed rate by executing the following calculations. In other words, the relative feed rate determining unit 8 divides the target feed rate given from the control parameter setting unit 7 by a difference between the reference voltage and the short circuit reference voltage to obtain a proportional constant. Next, the relative feed rate determining unit 8 calculates a difference (hereinafter, simply referred to as error voltage) between the measured average voltage and the reference voltage, multiplies this error voltage by the proportional constant, and obtains a value as a compensation feed rate. Lastly, the relative feed rate determining unit 8 adds the target feed rate to this compensation feed rate to decide a relative feed rate.
The relationship between the relative feed rate and the measured average voltage, obtained through the above-mentioned calculations, is collectively shown in the graph of FIG. 9. In other words, the relative feed rate is made to be proportional to the difference between the measured average voltage and the short circuit reference voltage, and the relative feed rate is set to be equal to the target feed rate when the measured average voltage is equal to the reference voltage.
However, the above-mentioned conventional art has a problem such that the value of the proportional constant is decided uniquely by the values of the target feed rate and the reference voltage. In other words, the proportional constant is a constant to determine the degree in which the relative feed rate is changed in response to variations in the measured average voltage, that is, the constant by which control gain is decided, so that this is the most basic and essential constant to define the machining control characteristic. Nevertheless, the constant is uniquely decided by the target feed rate and the reference voltage that are decided according to the settings of a machining plate thickness and the machining power supply 5, and therefore there is a problem that desired control characteristics cannot be set freely. Moreover, when the measured average voltage is not less than the short circuit reference voltage, the control gain becomes constant in all the areas. Therefore, if the response property is intended to be improved by increasing the proportional constant in the vicinity of the reference voltage in order to increase the machining speed, the amount of overshoot is inevitably increased as well. Thereby, the measured average voltage tends to easily go below the short circuit reference voltage in the vicinity of the short circuit reference voltage. As a result, the relative feed rate becomes frequently negative to repeat short circuit and opening, thus the machining state may become quite unstable.
In order to solve these problems, a modified technique has been proposed in which a voltage that makes the relative feed rate zero is set to be higher than the short circuit reference voltage. In this modified technique, the proportional constant is obtained not through calculation of the target feed rate and the reference voltage, but through multiplication of the error voltage by this proportional constant using the preset value to set a value as a compensation feed rate. Further, the target feed rate is added to this compensation feed rate to decide the relative feed rate. However, when the compensation feed rate is negative with its absolute value being greater than the target feed rate, the relative feed rate obtained through the calculations becomes negative, and in this case, zero is set as the relative feed rate. The reason why the relative feed rate is set to zero when the relative feed rate obtained through the calculations is negative, is explained as follows. When the relative feed rate is set negative, the wire electrode 1 backs up on the path. The wire electrode 1, which has once backed, again advances the same path, i.e., the path that has been once machined when a positive and relative feed rate is subsequently set. In this case, the side face of the workpiece 2 is machined, resulting in an excessively machined state. Therefore, the wire electrode 1 is allowed to back only when the backing is inevitably required, such as a case in which short circuit is occurring. In another cases, the wire electrode 1 is stopped to wait for the recovery of the state, which makes it possible to achieve far better machining quality even if the measured average voltage becomes low.
The relationship between the relative feed rate obtained through the above-mentioned modified technique and the measured average voltage is collectively shown in the graph of FIG. 10. In other words, in the range (a) of FIG. 10 in which the relative feed rate is positive, the relative feed rate is increased and decreased in proportion to the error voltage, and the relative feed rate is controlled to be equal to the target feed rate when the measured average voltage is equal to the reference voltage. In the range (b) of FIG. 10 in which the calculated relative feed rate is negative, the relative movement between the wire electrode 1 and the workpiece 2 is stopped. Moreover, when the measured average voltage goes below the short circuit reference voltage, the wire electrode 1 is allowed to back at a high speed from the workpiece 2.
The application of such a modified technique makes it possible to set the control gain independent from the target feed rate and reference voltage. Therefore, it becomes possible to control machining by setting appropriate proportional constants according to various machining conditions such as the setting of the machining power supply 5, the material and diameter of the wire electrode 1, the material and plate thickness of the workpiece 2 and the offset amount between the wire electrode 1 and workpiece 2. Moreover, since the relative feed rate becomes zero in the vicinity of the short circuit reference voltage, the measured average voltage hardly goes below the short circuit reference voltage, thereby making it possible to stabilize the machining state.
In the conventional modified technique, however, the machining volume at the time of finishing needs to be increased, and therefore a great amount of time is required for machining. This problem will be explained with reference to the figures as follows. FIG. 11 is a schematic diagram that shows how finishing machining is carried out in the wire discharge machining, FIG. 12 is a schematic diagram that shows a case in which there is waviness on the previously machined surface, and FIG. 13 is a schematic diagram that shows the profile of the machined surface after the finishing by applying the conventional modified technique to the previously machined surface with the waviness.
As shown in FIG. 11, in the wire discharge machining, the previously machined surface 100 having a greater surface roughness is subjected to finishing with smaller energy to form a currently machined surface 101 having a smaller surface roughness. Normally, great waviness, derived from various external disturbances, exists on the previously machined surface 100 having a greater surface roughness.
As shown in FIG. 12, even when the wire electrode 1 comes to a peak 100a of the waviness, the application of the conventional modified technique makes it possible to prevent the occurrence of short circuit since the relative feed rate abruptly drops to zero. However, since the machining with the wire electrode 1 being stopped progresses until the peak 100a of the waviness is removed, a recessed portion 101a tends to be formed at a portion corresponding to the rear side of the peak 100a of the waviness used to exist on the previously machined surface 100, on the currently machined surface 101 that is formed through the current finishing, as shown in FIG. 13.
Since the depth of the recessed portion 101a formed on the currently machined surface 101 is small in comparison with the height of the peak 100a that exists on the previously machined surface 100, it is certain that the degree of the machined surface roughness gradually reduces through the finishing machining. However, in the next finishing machining, it is necessary to carry out machining deeper than the recessed portion 101a, and therefore the machining volume for the depth from the currently machined surface 101 to the subsequently machined surface 102 becomes inevitably greater as shown in FIG. 13. Moreover, it is difficult to predict how deep the recessed portion 101a newly formed on the currently machined surface 101 will be with respect to the height of the peak 100a that used to exist on the previously machined surface 100. The machining volume performed in the next finishing machining, that is, the distance between the currently machined surface 101 and the subsequently machined surface 102 needs to be set with a greater margin. This also causes an increase in the machining volume.
After the finishing machining is performed, a reverse phenomenon as follows is generally experienced in the wire electric-discharge machining. Specifically, the reverse phenomenon in the machining volume is such that an insufficiently machined portion on the previously machined surface 100 is subjected to an excessive machining, while an excessively machined portion is subjected to an insufficient machining. In particular, as attempted in recent years, when the previously machined surface 100 having a great surface roughness is machined by using very small energy so that the number of finishing steps is minimized as small as possible to improve the total machining speed, the above-mentioned reverse phenomenon comes to appear more frequently, causing a great adverse effect on the machining speed.
Moreover, in the above-mentioned conventional modified technique, when the wire electrode 1 and the workpiece 2 are in contact with each other through machining dusts, the measured average voltage is not allowed to rise because a short circuit current actually flows through the machining dusts due to imperfect short circuit although the measured average voltage is not below the short circuit reference voltage. This case corresponds to the state in the range (b) shown in FIG. 10, in which the wire electrode 1 is being stopped and thereby the machining does not progress at all.
It is an object of this invention is to provide a wire electric-discharge machining apparatus capable of desirably setting a control gain so as to stabilize the machining state, and of improving the machining speed.
The wire electric-discharge machining apparatus according to one aspect of this invention comprises a relative feed rate determining unit that decides a relative feed rate between a wire electrode and a workpiece based on a measured voltage between the wire electrode and the workpiece. This wire electric-discharge machining apparatus carries out machining on the workpiece by generating a discharge between the wire electrode and the workpiece and relatively moving the wire electrode and the workpiece at the relative feed rate decided by the relative feed rate determining unit. The relative feed rate determining unit outputs a target feed rate as a target value when the measured voltage is equal to a preset reference voltage, and outputs a positive relative feed rate according to preset conditions, when the measured voltage exceeds a predetermined short circuit reference voltage but is below a switching voltage that is preliminarily set between the short circuit reference voltage and the reference voltage.
According to the above aspect, when the measured voltage is equal to the reference voltage, the wire electrode and the workpiece move at the target feed rate, and when the measured voltage exceeds the short circuit reference voltage, the wire electrode and the workpiece always move at the positive relative feed rate.
The wire electric-discharge machining apparatus according to another aspect of this invention comprises a relative feed rate determining unit that decides a relative feed rate between a wire electrode and a workpiece based on a measured voltage between the wire electrode and the workpiece. This wire electric-discharge machining apparatus carries out machining on the workpiece by generating a discharge between the wire electrode and the workpiece and relatively moving the wire electrode and the workpiece at the relative feed rate decided by the relative feed rate determining unit. When the measured voltage exceeds a predetermined short circuit reference voltage, the relative feed rate determining unit outputs a positive relative feed rate in which an amount of change per unit voltage change becomes smaller as a difference between the measured voltage and the short circuit reference voltage decreases.
According to the above aspect, when the measured voltage exceeds the short circuit reference voltage, the wire electrode and the workpiece always move at the positive relative feed rate in which an amount of change per unit voltage change becomes smaller as the difference between the measured voltage and the short circuit reference voltage decreases.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.